This invention relates to high voltage operational amplifiers, and, in particular, to a high voltage operational amplifier (OP-AMP) employing bipolar and high voltage insulated-gate field-effect transistors (IGFETs).
A problem exists when it is desired to design and manufacture a high voltage, low noise, high frequency operational amplifier (OP-AMP) with low input voltage offset. This is best explained with reference to FIG. 1 which shows a first, differential input, stage comprised of P-type IGFET transistors T1 and T2 having their sources connected in common to a common node 11 and via a current source 17 to a point of operating potential 19. The drains of transistor T1 and T2 are respectively coupled to the input 81 and output 82 of a current mirror amplifier (CMA) 83 comprised of N-channel double diffused metal oxide semiconductor (DMOS) transistors (T4, T5). The output of the first stage, which corresponds to the output of the CMA 83, is applied to the gate of an N-channel DMOS transistor (T3) which functions as the second stage of the operational amplifier. The use of the N-channel DMOS transistors for the CMA transistors is suggested by their high voltage capability and a manufacturing process similar to that used to form the output transistor T3 and the input differential transistors, T1,T2. However, the noise performance of the combination is very poor. Test results indicate that the equivalent input noise is in the range of a few thousand nanovolts per root Hertz. Such a high value for the equivalent input noise is unacceptable for a "low noise", high quality OP-AMP and negates the use of the DMOS transistors in the first stage of a high-quality OP-AMP.
The noise problem resulting from using high voltage DMOS transistors in the front end of an OP-AMP may be resolved by using a circuit of the type shown in FIG. 2 in which the N-channel DMOS transistors are replaced with NPN bipolar transistors. In FIG. 2, the current mirror 84 includes bipolar transistors Q1, Q2 and the second stage (QOUT) also consists of a bipolar transistor. The combination has a very acceptable noise performance. However, the output bipolar transistor QOUT has limited high voltage capability which limits the operating and output voltage of the circuit.
It is an object of the invention to overcome the limitation on high noise and/or high voltage operation present in the prior art circuits and to form an OP-AMP capable of operating at high voltages with low noise and also with low input voltage offset.